Asexual vs Sexual Reproduction
Asexual reproduction produces offspring from a single parent by mitosis, so offspring are genetically identical (clones) to the parent. Common forms include binary fission (bacteria, amoeba), budding (Hydra, yeast), fragmentation (Spirogyra, some flatworms), and vegetative propagation in plants (runners in strawberry, tubers in yam, bulbs in onion). It is fast and requires no mate, but produces no genetic variation, which is a disadvantage in a changing environment.
Sexual reproduction involves fusion of a male gamete (sperm/pollen) and a female gamete (egg/ovule) by meiosis and fertilisation, producing genetically varied offspring. Variation is the main evolutionary advantage — it gives a population raw material for natural selection to act on.
Reproduction in Flowering Plants
A typical flower has four whorls: calyx (sepals, protect the bud), corolla (petals, attract pollinators), androecium (stamens — filament + anther, the male part producing pollen), and gynoecium (carpel — stigma, style, ovary containing ovules, the female part).
Pollination is the transfer of pollen from anther to stigma. Self-pollination occurs within the same flower or plant; cross-pollination occurs between flowers on different plants of the same species, aided by wind, insects, water, or animals. Cross-pollination increases variation and is generally favoured evolutionarily.
Fertilisation in flowering plants is a double fertilisation: one male nucleus fuses with the egg cell to form the diploid zygote (which becomes the embryo); a second male nucleus fuses with the two polar nuclei to form the triploid endosperm (food store for the developing embryo). After fertilisation, the ovule becomes the seed and the ovary becomes the fruit.
Worked example: if a pollen grain carries 2 haploid male nuclei (n) and fuses with an egg cell (n) and two polar nuclei (n + n), the resulting zygote is (diploid) and the endosperm is (triploid).
Reproduction in Animals
Animal sexual reproduction requires gametogenesis: spermatogenesis in the testes produces sperm, oogenesis in the ovaries produces ova, both by meiosis, halving the chromosome number so fertilisation restores the diploid number.
Fertilisation may be external (as in most fish and amphibians, where gametes are released into water) or internal (as in reptiles, birds, and mammals, where sperm is deposited inside the female). Internal fertilisation gives the zygote more protection and is usually paired with fewer offspring but greater parental investment.
Development after fertilisation may be oviparous (egg-laying, e.g. birds, most reptiles — embryo develops outside the mother using yolk as food), viviparous (live birth, e.g. most mammals — embryo develops inside the mother, nourished via a placenta), or ovoviviparous (eggs are retained and hatch inside the mother's body, e.g. some sharks and lizards, with no placental nourishment).
Why This Matters for Your Exams
Exam questions in this area almost always hinge on correctly pairing a term with its example and its consequence: asexual = clone = fast but no variation; sexual = variation = evolutionary advantage; double fertilisation = 2n zygote + 3n endosperm; oviparous/viviparous/ovoviviparous distinguished by where the embryo develops and what feeds it. Get those pairings exact and most questions on reproduction become straightforward recall.